The present invention relates to a long wavelength surface-emitting semiconductor laser device with an emission wavelength of 1.3 to 1.6 micrometers; and, more particularly, to a high efficiency of Zn diffusion long wavelength surface-emitting laser has the ability to prevent an absorption loss in a resonator and effectively adjust a current flow.
The long wavelength surface-emitting laser is very available as a light source for short and long distance communications. In particular, such long wavelength surface-emitting laser with a high optical fiber coupling efficiency, a low unit cost by wafer-based fabrication, two-dimensional array characteristics and so on may be contemplated as a major light source for next generation optical communications and signal processing.
Since, however, the long wavelength surface-emitting laser has a lower energy of oscillation wavelength compared to a surface-emitting laser with an emission wavelength of 850 nm and 980 nm, it suffers from drawbacks including a high absorption loss due to a low quantum efficiency, a doping and so on, a low thermal conductivity of component materials and a structure fabrication for a high efficiency of uniform current induction.
A description of a structure of the conventional long wavelength surface-emitting laser and a problem thereof will be given with reference to FIG. 1.
FIG. 1 is a cross sectional view setting forth the structure of the conventional long wavelength surface-emitting laser grown in a general internal defect-free monolithic structure. As shown in FIG. 1, formed on an InP semiconductor substrate 11 for growing a laser structure with an emission wavelength of 1.3 to 1.6 xcexcm is an n-type bottom mirror layer 12; an active layer 13 covering a portion of the surface of the n-type bottom mirror layer 12; a p-type top mirror layer 14 disposed on top of the active layer 13, making a laser column; an insulating film 15 for covering a side of the active layer 13 and the p-type top mirror layer 14, and the n-type bottom mirror layer 12 which is not covered with the active layer 13 and a first metal electrode 16 connected with the p-type top mirror layer 14. A second metal electrode 17 is connected with the reverse side of the InP semiconductor substrate 11.
Each of the p-type top mirror layer 14 and the n-type bottom mirror layer 12 is a multi-layered semiconductor Distributed Bragg Reflector (xe2x80x9cDBRxe2x80x9d) doped with p and n type impurities, respectively, which is composed of a multi-layered structure implemented by alternatively growing two materials having different indices of refraction, and an InAlGaAs/InAlAs or InGaAsP/InP lattice matched to InP. The active layer 13 is composed of a quantum well structure adapted for a long wavelength with an oscillation wavelength of 1.3 xcexcm to 1.6 xcexcm.
The application of current between the first and second metal electrodes 16 and 17 in the long wavelength surface-emitting laser with the structure shown in FIG. 1 allows a current to be induced through an etched laser column, resulting in a gain of the active layer 13 thereby undergoing an oscillation.
For the current application, each of the p-type top mirror layer 14 and the n-type bottom mirror layer 12 are doped with p and n type impurities so that injection of hole and electron is possible. The n-type bottom mirror layer 12 and the p-type top mirror layer 14 are composed of a multilayer of heterojunction, causing the discontinuity of valence and conduction bands in interface. Thus, high doping concentration ranging from 1xc3x971018 cmxe2x88x923 to 5xc3x971018 cmxe2x88x923 is necessary to eliminate a resistance. Unfortunately, free carrier absorption rapidly increases with a higher concentration, resulting in a suddenly increased threshold gain for laser oscillation. Decreasing the doping concentration to eliminate the free carrier absorption loss results in an increased series resistance, increasing the threshold voltage to thereby generate a high thermal, which, in turn, rapidly decreases the oscillation efficiency.
Such a problem, i.e., an increase in resistance due to the heterojunction and an increase in absorption loss due to the doping, may be mostly occurred at the p-type top mirror layer 14 in which a current flows by the hole. The problem results in degraded characteristics of threshold current, output and operation temperature.
On the other side, in the structure of the long wavelength surface-emitting laser shown in FIG. 1, since a current is induced by the column formed by etch, it suffers from drawbacks including a scattering loss in the etched surface and a surface recombination loss in surface of the active layer by the current injection.
It is, therefore, a primary object of the present invention to provide a high efficiency of long wavelength surface-emitting laser device and method, in a resonator of long wavelength surface-emitting semiconductor laser device with an emission wavelength of 1.3 to 1.6 micrometers, which is capable of eliminating an absorption loss of a p-type doped layer and reducing a scattering loss in a mirror layer and a carrier loss due to a current induction.
In accordance with one aspect of the present invention, there is provided a surface-emitting laser device, comprising: a first conductive type of semiconductor substrate; a bottom mirror layer formed on the semiconductor substrate and composed of a first conductive type of semiconductor layer; an active layer formed on the bottom mirror layer; an electron leakage barrier layer formed on the active layer and having an energy gap larger than the active layer; a current induction layer formed on the electron leakage barrier layer and a second conductive type of semiconductor layer; a current extension layer formed on the current induction layer and composed of the second conductive type of semiconductor layer; and a top mirror layer formed on the current extension layer, wherein the top mirror layer includes undoped center portion and its both end having the second conductive type of dopant diffusion region.
In accordance with another aspect of the present invention, there is provided a method for manufacturing a surface-emitting laser device, comprising: a first conductive type of semiconductor substrate; a bottom mirror layer formed on the semiconductor substrate and composed of a first conductive type of semiconductor layer; an active layer formed on the bottom mirror layer; a current extension layer formed on the active layer and composed of the second conductive type of semiconductor layer; and a top mirror layer formed on the current extension layer, wherein the top mirror layer includes undoped center portion and its both end having the second conductive type of dopant diffusion region.
In accordance with still another aspect of the present invention, there is provided a method for the fabrication of a surface-emitting laser device, comprising the steps of: (a) forming a bottom mirror layer composed of a first conductive type of semiconductor layer on a first conductive type of semiconductor substrate; (b) disposing an active layer on the bottom mirror layer; (c) sequentially forming on the active layer an electron leakage barrier layer, a current induction layer and a current extension layer each of which is composed of a second conductive type of semiconductor layer; (d) forming a top mirror layer composed of an undoped semiconductor layer on the current extension layer; (e) forming a mask pattern on the top mirror layer; (f) selectively etching a portion of the top mirror layer using the mask pattern as an etch mask to thereby form a laser column composed of the top mirror layer; (g) forming the second conductive type of dopant diffusion layer at both ends of the top mirror layer using the mask pattern as an etch mask; (h) etching the top mirror layer and the current extension layer using the mask pattern as the etch mask, until the current induction layer is exposed; (i) wet etching the current induction layer to form a pattern which overlaps with the top mirror layer at between the second conductive type of dopant diffusion layers; and (j) removing the mask pattern.
In accordance with still another aspect of the present invention, there is provided a method for the fabrication of a surface-emitting laser device, comprising the steps of: (a) forming a bottom mirror layer composed of a first conductive type of semiconductor layer on a first conductive type of semiconductor substrate; (b) disposing an active layer on the bottom mirror layer; (c) forming a current extension layer composed of a second conductive type of semiconductor layer on the active layer; (d) forming a top mirror layer composed of an undoped semiconductor layer on the current extension layer; (e) forming a mask pattern on the top mirror layer; (f) selectively etching a portion of the top mirror layer using the mask pattern as an etch mask to thereby form a laser column composed of the top mirror layer; (g) forming the second conductive type of dopant diffusion layer at both ends of the top mirror layer using the mask pattern as an etch mask; (h) etching the top mirror layer and the current extension layer using the mask pattern as the etch mask, until the active layer is exposed; (i) wet etching the active layer to form a pattern which overlaps with the top mirror layer at between the second conductive type of dopant diffusion layers; and (j) removing the mask pattern.